The principal enzymes of the glycogenolytic pathways: phosphorylase, phosphoglucomutase and amyloglucosidase have been assayed in the mantle tissue of Mytilus edulis L. throughout the period of gametogenesis and glycogen deplection (January-April, 1980). Phosphorylase and amyloglucosidase activities were in the ranges 0.14-0.30 and 0.05-0.30 µmol/min per g wet weight, respectively. Phosphoglucomutase activity was ten times as high as that of the other two enzymes and the mass action ratio data indicated that the enzyme was near equilibrium. It is suggested that during gametogenesis, glycogen is broken down by both hydrolytic and phosphorolytic mechanisms.